The present invention relates to methods for controlling the delivery of a reductant for oxides of nitrogen and compressed air to an exhaust gas produced by a combustion engine.
Nitrogen monoxide and nitrogen dioxide, collectively referred to as oxides of nitrogen or xe2x80x9cNOxxe2x80x9d, are commonly cleaned from the exhaust gases produced by internal combustion engines using catalysts. In addition to removing NOx, these catalysts also remove unburned hydrocarbons (HC) and carbon monoxide (CO). When the engine is operated with a lean air/fuel ratio, the catalyst is efficient at removing the HCs and COs because of the extra oxygen in the exhaust gas. However, the extra oxygen tends to inhibit the removal of NOx. Conversely, when the engine is operated with a rich air/fuel ratio, NOx removal efficiency of the catalyst is increased but the HC and CO removal efficiency is decreased.
Designers have focused their attention in the past several years to an approach of mixing a reductant with the exhaust gas upstream of the catalyst. The presence of the reductant in the catalyst improve the NOx reduction efficiency. Furthermore, this improvement can be made in the presence of excess oxygen output from a lean burning engine, including diesel engines. For good NOx reduction efficiency, it is necessary that the reductant be thoroughly mixed with the exhaust gas. Two methods have been used to atomize a fluid reductant, pumping the fluid through a spray nozzle, and injecting the fluid into a stream of compressed air that is sprayed into the exhaust gas.
U.S. Pat. No. 5,645,804, issued to Sumiya et al. on Jul. 8, 1997, discloses three embodiments of a system that mixes a hydrocarbon reductant with the exhaust gas. In the first embodiment, a compressed air source pumps air into the exhaust gas by way of a funnel-shaped nozzle situated in the exhaust pipe. The reductant, in liquid form in a storage tank, is drawn into the funnel-shaped nozzle by the venturi effect. Control of the reductant flow rate is achieved by controlling the pressure inside the storage tank, controlling the pressure of the compressed air, or by controlling the flow rate of the compressed air. Reductant atomization is provided by the reductant entering the compressed air stream inside the funnel-shaped nozzle. Consequently, atomization effectiveness varies with the speed of the air flow in the funnel-shaped nozzle.
In the second embodiment disclosed by Sumiya et al., the hydrocarbon reductant is pumped directly from its storage tank into a spray nozzle situated inside the exhaust pipe. Control of the reductant flow rate is provided by a throttle valve. Atomization is provided by the tip of the nozzle.
The third embodiment disclosed is similar to the second embodiment with the addition of compressed air injected into the reductant just prior to the spray nozzle.
In both the second and third embodiments, the effectiveness of the reductant atomization varies with changes in the pressure differential across the tip of the nozzle.
The present invention provides a method for controlling an air source and a reductant source that delivers compressed air and a reductant respectively to a mixer. From the mixer, the compressed air""s pressure forces the reductant through a nozzle and into an exhaust gas at a position upstream from a catalyst. The present invention provides a reductant control signal to the reductant source causing a calculated quantity of reductant per second to be delivered to the mixer. An air control signal is provided to the air source causing the compressed air pressure to be a predetermined value above the exhaust gas pressure. Maintaining a constant differential pressure across the nozzle provides good reductant atomization under all engine operating conditions.
Control of the air source is performed in closed-loop. A differential pressure error value is derived from an actual differential pressure across the nozzle and the predetermined nozzle differential pressure. Next, this differential pressure error value is transformed into the air control signal. Transformation may include integral and proportional terms. To limit oscillations, the transformation could range from slightly underdamped to overdamped and/or provide a deadband around a zero error for the differential pressure error value.
The reductant source control method calculates the desired reductant flow rate based upon the engine speed, engine load, catalyst temperature, gas space velocity flowing through the catalyst and the air pressure inside the mixer. The desired reductant flow rate is then transformed into the reductant control signal. This transformation may be accomplished in two steps. In the first step, the reductant control signal is calculated assuming that the reductant source sees a predetermined reference pressure inside the mixer. In the second step, the reductant control signal is adjusted up or down based upon the actual air pressure inside the mixer being higher or lower than the predetermined reference pressure respectively.
Alternative embodiments of the present invention include failure detection and correction methods. Detectable failures include the air supply""s inability to produce the necessary compressed air pressure, and a clogged nozzle. These failures are detected by the air control signal and the actual nozzle differential pressure passing through respective thresholds in opposite directions. Failure corrections may includes outputting a failure signal, stopping the flow of reductant to the mixer, and stopping the flow of the air stream to the mixer
Accordingly, it is an object of the present invention to provide a method for controlling delivery of compressed air to a nozzle to maintain a constant differential pressure across the nozzle, and control delivery of a reductant for oxides of nitrogen into the nozzle such that a calculated flow rate of the reductant is sprayed through the nozzle into the exhaust gas created by a combustion engine.